It has been well known to detect interfaces between two liquids such as oil and water or water and air by ultrasonic methods. Exemplary of such art is U.S. Pat. No. 3,520,186 issued to G. L. Adams et al. July 14, 1970. However, such prior art does not afford suitable techniques for measuring the distance between two interfaces such as for example to indicate the thickness of a layer of oil on top of water where there is on one side on oil-water interface and on the other side an oil-air interface.
In many operations in the petroleum industry it becomes necessary to measure the thickness of an oil layer floating on a body of water. These include oil tanks on tankers where sea water is commonly pumped into and out of tanks for ballast purposes. A similar application on a tanker is a settling or separation tank, from which separated water is pumped overboard.
A similar need to measure an oil layer thickness arises on the high seas in the event of accidental discharge of oil or as a result of ship collisions. Some machines whose function it is to clean up spilled oil in rivers, harbors, coastal regions, near refineries, etc., require a layer thickness sensor to function properly.
Deficiencies of the prior art techniques for this purpose are accentuated by various technical problems presented in measurement by ultrasonic sensing of the thickness of layers of oil on water for example. Perhaps the most important problem to be overcome is the necessity to work with a very large range of signal amplitudes, so that sensitive equipment is necessary to process weak signals which therefore is readily overloaded with strong signals. The interface between water and oil provides a weak reflection of ultrasonic pulses that must be detected and processed reliably in the presence of much larger signal amplitudes produced for example at the oil and air interface. Furthermore under dynamic conditions where the surfaces are changing such as when oil is being cleaned off the water surface or there is wave activity, successive signal pulses may encompass a very wide range of signal amplitude variations ranging from noise levels upward.
A further significant deficiency in the prior art is the signal to noise improvement factor necessary to provide meaningful measurements. Bubbles can give false signals or absorb signal strength for example. Waves can cause reflection angles to change with resultant signal deterioration. Dynamic film conditions can cause variations of thickness that may need be distinguished from noise conditions.
Even in the display of measurements many heretofore unsolved problems exist. The resolution need be good enough to distinguish signals from noise, and to display very thin film thicknesses with accurate dimensioning. Preferably such display to be on the same display medium that will handle a wide range of thickness dimensions automatically without intervention or adjustment by the operator.